Superconductor/insulator (s-i) layer structures are of interest in a variety of applications. For instance, it has long been recognized that the performance limits of some categories of electronic equipment (e.g., signal processors, computers) could be substantially improved if their circuitry could comprise superconductive tunnel junctions (STJs, frequently referred to as "Josephson" junctions). As is well known, STJs comprise a superconductor/insulator/superconductor (s-i-s) layer structure. For a review of STJs see, for instance, "Applied Superconductivity", Vol. 1, V. L. Newhouse, editor, Academic Press, 1975, incorporated herein by reference.
Prior to 1987 essentially all of the relevant effort was directed towards STJs which used metal (generally elemental metal or alloys, e.g., Pb, Nb, Pb-In-Au alloy) electrodes. However, the prior an technology has not found significant commercial use, at least in pan due to the fact that all variants of the technology have operating temperatures below 10K. Recent advances in cryogenic engineering have resulted in commercially available closed-cycle refrigerators which make temperatures down to about 10-12K readily and conveniently accessible. However, temperatures below 10K still can only be reached through use of liquid He. Liquid He not only is relatively expensive but, probably more importantly, is rather inconvenient to use, requiring special facilities and techniques.
Starting in 1986, superconductors were discovered that had critical temperatures (T.sub.c) higher than previously known superconductors. Most of these "high T.sub.c " superconductors were cuprates. Exemplary are La-Ba-Cu-oxide, Y-Ba-Cu-oxide, Tl-Ba-Ca-Cu-oxide, and Bi-Pb-Sr-Ca-Cu-oxide. New non-cuprate oxide superconductors were also discovered. See U.S. Pat. No. 4,933,317, which discloses a superconductor of nominal composition ABiO.sub.3 (with A being Ba and at least one monovalent element, exemplarily K), and R. J. Cava et at., Nature, Vol. 332, pp. 814-816, Apr. 28, 1988, both incorporated herein by reference. It is now known that material of composition Ba.sub.1-x K.sub.x BiO.sub.3 (x.about.0.4) can have T.sub.c .about.30K.
To date most high T.sub.c -related development efforts have been directed towards cuprates and, in particular. towards cuprates having T.sub.c &gt;77K. If STJs of good quality could be reliably manufactured in a cuprate system with T.sub.c &gt;77K then cooling could be readily accomplished with liquid N.sub.2, or with a simple closed-cycle refrigeration system. However, even though tunnel junctions have been manufactured in YBa.sub.2 Cu.sub.3 O.sub.7 and other high T.sub.c cuprates (see, for instance, J. M. Valles et al., Materials Research Society Symposium Proceedings, Vol. 169, pp. 983-986 (1990)), the behavior of these junctions typically departs substantially from that of an "ideal" BCS tunnel junction. In particular, these junctions frequently exhibit a relatively low (even zero) "Josephson" current, a substantial conductance at zero bias, and a tunneling density of states which is untypical of other superconductors.
In view of these characteristics it appears doubtful whether commercially useful circuits that comprise high T.sub.c cuprate-based s-i layer structures (e.g., STJs) will be reliably manufacturable. On the other hand, it would clearly be highly desirable to have available a technology that can reliably produce s-i layer structures that can, for instance, yield STJs that exhibit substantially ideal behavior at temperatures that can be reached without use of liquid He. This application discloses such a technology.